n=int(input()) H=list(map(int,input().split())) X=10**10 q=[(1,X,0)] a=0 for i in range(n): h=H[i] while 1: l,r,f=q.pop() h-=r-l+1 if f: a-=r-l+1 if h<0: break l,r=r+h+1,r q+=[(l,r,f)] if f: a+=r-l+1 h=H[i] l,r,f=1,h,(i+1)%2 q+=[(l,r,f)] if f: a+=r-l+1 print(a)